Various systems have been used in the past as tensioning devices for producing tension in chains. For example, the tensioning device shown in Japanese Unexamined Utility Model Application Publication S52-34377 consists of a cam plate having a cam surface which abuts the chain, and a spring which latches on the cam plate and causes the cam plate to turn so that the cam surface runs into the chain when the chain undergoes elongation.
Furthermore, the tensioning device shown in Japanese Unexamined Utility Model Application Publication S52-99478 consists of a tensioner arm on which the chain slides, a cam plate having a cam surface which abuts the back surface of the tensioner arm, and a spring which causes the cam plate to turn so that the cam surface runs into the back surface of the tensioner arm when the chain undergoes elongation.
However, with the tensioning devices disclosed in the publications mentioned above, when the can surface runs into the chain or tensioner arm during chain elongation, the amount of elastic deformation of the spring decreases, and therefore the spring force decreases. Consequently, the pressing force from the spring which acts on the chain through the cam is also reduced.